1. Field of the Invention
The present invention relates to an electrode plate suitable for use in alkali storage batteries such as nickel-cadmium, nickel-zinc and nickel-hydrogen batteries and in a secondary Li battery.
2. Description of the Prior Art
Lead and alkali storage batteries are used as power sources in various fields. Of these, a small-size alkali storage battery has widely been used in various portable devices and a large-size alkali storage battery in industrial uses because of the anticipated high reliability of the alkali storage batteries and because of the possibility of miniaturization and weight reduction thereof. In this alkali storage battery, the negative electrode is composed of not only cadmium but also zinc, iron or hydrogen. However, the positive electrode is mostly composed of nickel although positive electrodes respectively composed of air and silver oxide have been studied in some proposals. The use of the alkali storage battery has been expanded in accordance with the improvement of properties attained by the change from the pocket type to the sintering type and in accordance with the realization of sealing.
However, in the conventional powder sintering method, an increase of the plate porosity to 85% or higher leads to a substantial deterioration of strength, so that the filling of the active substance encounters a limit. Thus, the increase of the battery capacity encounters a limit. In substitution for the sintered plate, foamed or fibrous plates each having a porosity increased to 90% or higher have been proposed, some of which have been put into practical use. In this connection, processes have long been proposed in which a foamed resin is coated with a metal paste and sintered to thereby obtain a sintered body having a high porosity. For example, Japanese Patent Publication No. 17554/1963 disclosed a process comprising rendering metal powder muddy, impregnating a urethane foam with the muddy metal powder, carbonizing the resin in hydrogen and converting the metal to the half molten state, thereby obtaining a porous metal body.
For example, a porous metal body having a porosity as high as at least 90% can be produced by providing powdery nickel as metal powder in the form of a paste, impregnating a foamed resin with the paste and conducting sintering in hydrogen, thereby enabling produbtion of a high-capacity electrode as a battery base. However, the resultant porous metal body is poor in mechanical strength, for example, tensile strength because of the high porosity and because of the absence of a porous core material such as punching metal employed in the conventional powder sintering product.
Therefore, when the porous metal body is filled with an active substance in a hoop-like form, it has the danger of being deformed or broken by the load applied during carriage thereof. Further, the electric resistance of the resultant base is slightly increased, so that a voltage drop is caused to occur especially at the time of high discharge.
Generally, a terminal strip is secured to the electrode by spot welding. The high porosity and the absence of any core material increase the probability of breakage of the spot-welded part. Further, in the method known as the tabless method in which a circular terminal is welded to an upper part of an electrode with a cylindrical structure, the upper part of the electrode has such a poor strength that this arrangement can hardly be employed.
In a large-size battery for use in, for example, an electric vehicle, its large electrode surface area increases the electric resistance of the electrode plate and hence increases the voltage drop at the time of high-rate discharge, so that the battery encounters a limit in available output. Moreover, a voltage distribution occurs in the electrode surface with the result that the charge efficiency is deteriorated.
Although the contribution of the use of the porous Ni body as an electrode plate of battery toward the enhancement of the battery capacity is great as mentioned above in the prior art section, metallic nickel is expensive and further its resource has a problem since the consumption thereof is foreseen to be enormous when the above alkali storage batteries are employed in electric vehicles, etc. in the future.